Previous research has shown that the risk of sudden death due to cardiac arrhythmias can be predicted by observing the shape of recorded endocardial electrograms in response to pacing.
The diagnostic change in electrograms consists of small deflections in the recorded electrogram following early stimulation of the heart. The heart is stimulated with apparatus that generates a stimulation sequence at one site in the heart and records electrograms from other sites within the heart.
The above technique has been used to form intra-ventricular conduction curves in which the delay of each potential within an electrogram following an early stimulation of the heart is plotted against the interval between the stimulus and the last stimulus of a constant rate sequence of stimuli. Typically this involves between 230 and 250 early stimuli with the interval at which each early stimulus is applied is reduced by one millisecond on each occasion. These curves have characteristic shapes that can be analyzed to estimate the risk of sudden death. This technique is described in GB2439562.
In previous research using this technique, the diseases that affect the heart have been assumed to uniformly affect all of the tissue within the heart. In the case of patients that have recovered myocardial infarction, the abnormalities are confined to the tissue that surrounds the infarct. While it is possible to make detailed endocardial recordings in that region, such a technique is too complex for routine use, because for example, the need for multiple left ventricular electrodes.
A surface electrocardiogram (ECG) consists of potentials measured on the torso surface, these represent the weighted sum of all of the currents active within the hear. Surface ECGs are commonly used for diagnostic purposes.
The small deflections following an early stimulus can be detected using a surface ECG recording although the detected deflections are extremely small. Therefore a process needs to be carried out to distinguish these small deflections from noise.
A common technique is to average the signals to accentuate small potentials from the noise.
It is well known that some patients have small delayed potentials discernible in their average ECG during sinus rhythm and this has some association with the risk of sudden death.